DEVELOPMENT OF MOLLUSCS. 



199 



rangement. This gemmation, or, in other words, this 

 stem structure, still occurs in Echinoderms, inasmuch as 

 some species of star-fish possess such powers of repro- 

 duction as to enable a single arm or ray, when torn off, to 

 complete itself into a whole animal. Nay, Kowalewsky's 

 observations render it highly probable that the separa- 

 tion of rays, and their completion by gemmation, is in 

 some species a normal process. Haeckel's hypothesis is 

 thus laughed at only by those who are afraid to think or 

 reason. 



In the family of the Mollusca, the so-called navicula 

 larva testifies the kinship of at least two of the great 

 classes. The third and most advanced class, that of the 

 cuttle-fish, had perhaps lost their distinctive badge even 

 in those primaeval times when, under the somewhat lower 

 forms of the Tetrabranchiata, they left their shells in 

 the Silurian strata. But the bivalve shells, or Lamelli- 

 branchiata, and the snails, widely differing in anatomical 

 development, and constituting two natural classes, have 

 a common larval form, or, if the larvae display different 

 shapes, a highly distinctive common larval organ, the 

 velum. The accompanying diagram gives on the right 

 the navicula of a cockle-shell as seen from behind. At 

 the anterior end, two fleshy lobes have been formed, 

 edged with cilia, by the vibrations of which the young 

 animal, even in the egg, performs spiral twisting mo- 

 tions; in the midst of the cilia rises a little prominence, 

 furnished with a longer filament. These ciliated lobes or 

 vela,' merging into one another, are shown on the left in 

 the larva of a sea-snail (Pterotrachea), as seen nearly in 

 profile, and in the phase in which the eyes and auditory 

 apparatus, the foot and operculum, as well as a delicate 

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